1. Field of the Invention
The invention relates to a coupling circuit arrangement for data communication over power lines. Such a circuit arrangement is used to couple data to a power line for transmission over the power line and for receiving data may have been transmitted over the power line.
2. Brief Description of Related Developments
Communications networks are commonly used to transport digital data between devices and systems in an industrial, residential or transportation environment. The digital data can contain encoded information such as voice, video, control or status information. To transmit and receive this data some kind of physical transmission medium is required. Examples are RF, Coax, Ethernet twisted pair, telephone cables and power line cables.
In the prior art it has been suggested to use power lines to transmit and receive communication data. Examples are described in U.S. Pat. Nos. 5,241,283 and 5,452,344. The power line is a connectivity medium primarily used to transmit AC or DC power to an electronic or electrical circuit. Examples are the AC power distribution in a house or the DC voltage bus in an automobile. The power line medium is a favorable medium for data networking in these applications because it is a ubiquitous medium and the communication link implementation saves the cost of implementing additional connection cables.
Power line communication signals are typically transmitted on carrier frequencies in frequency bands allocated by national regulations. Within the allocated frequency band the transmitted digital signal is typically modulated using various schemes such as ASK (Amplitude Shift Keying), FSK (Frequency Shift Keying), Spread Spectrum techniques. The modulation function is typically performed by a modulator/demodulator integrated circuit (IC). This modulation/demodulation function may also form part of a microprocessor or microcontroller. The modulated transmit and receive signals are subsequently coupled onto the power line via a coupling circuit. In the case of power lines used to transmit AC power, the coupling circuit also has to filter out the AC power frequencies. In the case of a DC power line, the coupling circuit simply has to block the DC voltage of the power line.
Additionally in many cases the power line is distributing voltages levels, which can be considered potentially dangerous to life. To protect the user it is usual to galvanically isolate any circuitry electrically connected to the power line voltages. In some instances this may be achieved by means of the system housing or mechanical construction, but in many cases this is not a practical solution because the user has to have direct electrical access to the low voltage circuitry. In this instance it is usual to galvanically isolate the low voltage circuitry from the high voltage circuitry. In this instance the common method of achieving galvanic isolation is to use a transformer to couple the modulated signal onto the power line.
This transformer has to be able to couple a large signal current within the communication pass band. Additionally the construction of the transformer also has to meet the requirements of pertinent national and international safety requirements. These two factors together mean that the ability to reduce the physical dimensions and cost of the coupling transformer is limited.
Another problem with coupling low voltage circuitry onto the power line with a transformer is that high voltage surges or transients within the communication passband are directly coupled into the low voltage circuitry, potentially damaging not only the power line interface components but also interconnected circuitry. This problem is reduced with the use of surge protection devices however still represents a potential risk to overall system robustness.
Another method to provide galvanic isolation is to use multiple optocouplers to isolate the digital input/output between the modulator/demodulator IC and the system microcontroller. In this manner the power line modem is completely galvanically isolated from the system.
This method has the advantage that any surge transients on the power line only impinge on the power line modem components. The system low voltage circuitry is effectively protected isolating any potential surge damage just to the power line modem, significantly increasing overall system robustness.
This configuration however cannot be used in the case of system microcontrollers, which implement the modulator/demodulator function internally. In this case the microcontroller general purpose input/output ports typically need to be directly connected to the low voltage side circuitry.
Because the power line medium is intended primarily to transmit power it does not have ideal characteristics for transmitting or receiving modulated communication data, particularly in terms of line impedance and attenuation loss. To successfully transmit the modulated signal across the power line, the transceiver device must be capable of transmitting a relatively high signal power level. To satisfy this requirement, the output of the modulator circuit has to be buffered with an analogue line driver.
The analogue line driver function can be implemented in a number of ways. It can be implemented using a number of discrete components or using an integrated analog line driver IC. Alternatively it could be integrated within the modulator/demodulator IC. However, the integration of the line driver within the modulator/demodulator is impeded by the fact that the most suitable lowest cost IC technology for the modulator/demodulator function is not the most suitable for the voltage and power requirements of the line driver. Consequently combining the modulator/demodulator on the same IC results in compromises either in the function of the modulator/demodulator or the line driver circuit. For this reason commonly the line driver function is situated external to the modulator/demodulator IC.